1. Field of the Invention
The present invention relates to a photovoltaic system comprising a plurality of photovoltaic modules which are electrically connected to a photovoltaic generator whose first string end forms a negative pole and whose second string end forms a positive pole. The photovoltaic generator includes a plurality of photovoltaic modules which are connected to form at least one string. Multiple parallel-connected strings may also be provided.
2. Description of the Background Art
Systems of this type are all too common. In systems of this type, a number of photovoltaic modules, for example ten, are connected in series. The modules form a so-called string. Each photovoltaic module, in turn, includes, for example, 100 photovoltaic cells, which, for their part, are electrically connected in series. An individual semiconductor photovoltaic cell in common use today generates a voltage of approximately 0.5 volts when irradiated by solar energy, which generates a module voltage of 50 volts. When the strand is under load, a voltage of approximately 500 volts results, hereinafter referred to as the string voltage, depending on the system use. When operated without load, the string voltage increases to approximately 800 volts. It is common to combine a plurality of strings, e.g. 10 strings, by means of parallel connection, and then to provide the generated energy for further use with the aid of a common collecting line.
The generated electrical energy is present in the form of direct voltage. It is converted to alternating voltage with the aid of an inverter. The circuits illustrated by way of example in FIGS. 2 and 3 are currently common. The same components are provided with the same reference numerals in each case.
According to FIG. 1, a photovoltaic system 1 comprises a plurality of photovoltaic elements 3, which are connected in series and form two strings 5, which are parallel-connected to each other. Photovoltaic generator 6 formed in this manner has a first and a second string end 7, 9, which have negative potential P1 and positive potential P2, respectively. First string end 7 is the negative pole of photovoltaic generator 6 and thus has first (lower) string potential P1, and second string end 9 is the positive pole of photovoltaic generator 6 and thus has second (higher) string potential P2. An inverter 11 is connected to string ends 7, 9. Voltage Uo between string ends 7, 9 currently amounts to the approximately 500 V mentioned above under load and to the 800 volts mentioned above when operated without load. The insulation of the cables laid to the photovoltaic modules is designed for a value of approximately 1,000 volts, which is sufficient to safely operate the module types of this variant in common use today.
In the interest of simple representation, second circuit 1 illustrated in FIG. 2 includes only one string 5 of series-connected modules 3 in photovoltaic generator 6. This photovoltaic system 1 brings a disadvantage into play. In the illustrated so-called floating operation of photovoltaic system 1, approximately the same amount of voltage is connected to ground 13 at each of the two string ends 7, 9. Positive string potential P2 when operated without load (Uo=800V) is approximately +400 volts to ground 13, based on the example, and negative string potential P1 is approximately −400 volts to ground 13. These voltages to ground 13 occur despite the floating operation, due to a relatively small and negligible conductance (reciprocal value of the ohmic resistance) of the comparatively long connecting lines between modules 3 (wiring of system 1) and the supply lines to inverter 11. In an equivalent circuit diagram representation, the low conductance is symbolized by a resistance 14, which leads to ground 13 approximately in the center of series connection 5 of modules 3. As a result, parasitic discharges to ground 13 ultimately assume great dimensions, and the aforementioned potential distribution +400V, −400V to ground 13 arises in operation without load, since this is the most favorable distribution in terms of energy for overall system 1. Once again, no problems arise in the case of common cabling having an insulation safety value of 1,000 volts.
The provision of a constant voltage source which raises the negative pole of the photovoltaic system to a positive potential is known from DE 20 2006 008 936 U. This opposite procedure is carried out for entirely different purposes: The discharge of electrons from the TCO layer of module 3 is reduced so that cathode discharges are lowered or avoided entirely in order to prevent cathode erosion at the module.
The same measure is known from EP 2 086 020 A2, which corresponds to U.S. Publication No. 20090101191, and which is incorporated herein by reference. In the device for raising potential described therein, a further variant is provided, according to which the constant voltage source is connected at the positive pole of the photovoltaic system in order to raise this voltage to a higher potential for the purpose of reducing a danger of a lightning strike. Note that neither document addresses the lowering of potential, and such an approach would work against the desired objectives described therein.
The field of photovoltaics is currently undergoing technological changes which are aimed at more powerful modules having a higher output voltage than the aforementioned 50 volts for the on-load voltage (500 volts over the string of 10 modules) and 80 volts for the off-load voltage (800 volts for the string off-load voltage). Along with the developments on the inverter side is the ability to process these higher voltages. This is also based on the circumstance that higher voltages at the same power go hand in hand with lower currents, which is a positive trend where the cable cross sections are concerned. Existing cables in an existing photovoltaic system could therefore continue to be used, even after replacing old, i.e. obsolete, modules with new, modern modules, if the insulation requirements are still met.